The present invention relates, generally, to method and apparatus for cleaning the interior of casing used in oil and gas wells, and specifically, to methods and apparatus for cleaning such casing before completing the well.
The prior art has generally accomplished the cleaning of the interior of downhole casing, before completing the well, by running a string of drill pipe having a brush thereon and a drill bit at the end of such drill pipe to traverse the casing and drill out any obstructions in the casing, for example, cement and other debris, and with the brush enables the casing to be cleaned out. Because the internal diameter of the casing is smaller than the diameter of the uncased well which had just been drilled prior to the casing being cemented in, the drill string used to drill the hole must first be laid down and then a second string of drill pipe of smaller diameter and having a smaller diameter drill bit, is used to run through the casing. Laying down one string of drill pipe and running a different string of drill pipe, sometimes referred to as a xe2x80x9cwork stringxe2x80x9d, into the cased well to clean it out is very time consuming and is not cost effective.
In U.S. Pat. No. 4,896,720 to Mark W. DeRouen, there is disclosed an assembly which uses a brush attached to a top cement plug and a second brush connected between the upper cement plug and a lower cement plug used in the cementing operation. The assembly in intended to be pumped down the interior of the casing without using the work string of drill pipe and drill bit to clean out the interior of the casing. The brushes which are shown and described in U.S. Pat. No. 4,896,720 are fabricated from a nonrigid polyurethane foam and use a plurality of helical wraps of wire bristles disposed on the exterior surface of each such brush thereof, with the intent of the bristles contacting the interior surface of the casing. The purpose of the nonrigid polyurethane foam was an attempt to have the foam act as a spring-like device which would compress and maintain a constant contact with the interior wall of the casing. With such a device, however, the increasing pressures of the drilling fluid in the wellbore with depth causes the polyurethane foam to compress and pull the bristles away from the internal surface of the steel casing, thus resulting in the internal diameter of the casing not being thoroughly cleaned. Moreover, the brush body of the U.S. Pat. No. 4,896,720 requires an aluminum rod running along its length, from one end to the other, in an attempt to provide some degree of stability to the brush body, but which nonetheless fails to prevent the polyurethane foam from being compressed by the fluid pressure at the deeper depths encountered in a cased wellbore.
It should be appreciated that the combination brush and cement plug described in U.S. Pat. No. 4,896,720 is not as easily drillable as might be desired. Because the body of the polyurethane foam brushes, disclosed in the ""720 patent, are resilient, as a drill bit commences to drill out such brushes, the brushes will sometimes begin to rotate with the drill bit and delay the drilling out process by a considerable period of time. This type of problem occurs with attempting to drill out the resilient cement plugs themselves and is only worsened by adding in one or more brushes having resilient bodies with the identical problem.
It is therefore the primary object of the present invention to provide a new and improved method and apparatus for cleaning the interior of well casing, in which the bristles of the one or more brushes in the apparatus maintain a constant contact with the interior of the casing down to the desired depth to which the apparatus is pumped down.
As noted in U.S. Pat. No. 4,896,720, specifically in its Col. 3, lines 48-52, the density range for the nonrigid polyurethane foam brush body is preferably between eight and ten pounds per cubic foot. In sharp contrast, to avoid the polyurethane brush body being a compressible foam, the density selected for the present invention must be greater than ten pounds per cubic foot, preferably at least twelve pounds per cubic foot, if using polyurethane as the brush body.
It is also another object of the present invention to provide new and improved methods and apparatus for cleaning the interior of wellbore casing, in which the apparatus is more easily drilled out than with the methods and apparatus known in the prior art.
These and other objects, features, and advantages of the present invention will be better understood after reviewing the appended drawings and the following detailed specification.